This study addresses the following question: how prevalent is "chronic fatigue syndrome", or CFS, -- sometimes called "chronic mononucleosis" or "chronic Epstein-Barr virus (EBV) infection" -- and other causes of chronic fatigue, in the general population of adults and in adults seeking primary medical care for any reason? The general population surveyed will consist of a random sample of 4000 enrollees of a health maintenance organization (HMO) -- Group Health Cooperative of Puget Sound, Seattle WA -- regardless of whether these enrollees have ever sought medical car from the HMO. The primary care practice population will be drawn from a random sample of 1000 patients at Brigham and Women's Hospital, in Boston MA. Patients who declare that they have felt unusual fatigue or less of energy, either constantly or repeatedly, for at least the past six months, sufficient to interfere with their work or responsibilities at home, and who give informed consent, will be entered into the study. From each patient a detailed past medical history and history of current illness will be obtained; a complete physical examination will be performed; and a battery of laboratory tests will be performed according to an explicit protocol designed to pursue various elements of the differential diagnosis of chronic fatigue. The patients will be followed every six months for three years, with repeated medical history, physical examination and laboratory data obtained. From each patient, serologic studies of all six human herpesviruses -- including EBV and human B-lymphotropic virus (HBLV) or human herpesvirus 6 (HHV) -- will be obtained, because of pervious suggestions that primary infection with or reactivation of latent infection with these viruses is associated with CFS. Similar historical, physical examination and laboratory data will be obtained also from an age- and sex-matched group of control subjects. That will allow calculation of the sensitivity, specificity and predictive values of various clinical and laboratory features of CFS and other causes of chronic fatigue. It also will allow examination of the association of various human herpesviruses with CFS.